Saint-Rémy is at its best in May–June and September–October. The weather is warm enough for outdoor dining and walking, the Alpilles wildflowers (spring) or autumn colours make the countryside beautiful, and the village is busy but not overwhelmed. July and August are hot, crowded, and expensive — the Wednesday market swells to twice its usual size with tourists, accommodation prices spike, and restaurants need bookings weeks ahead.
Winter is genuinely quiet and has its own appeal. The village returns to its Provençal self; the plane tree avenues are bare but dramatic, and the Glanum site and Saint-Paul-de-Mausole are easily visited without crowds.
The Feria of Saint-Rémy, a Spanish-influenced bullfighting festival, runs in August and animates the town considerably. If the spectacle doesn't appeal, it's worth knowing about in advance so you can plan around it.